In you CSS, make sure that the bg image comes befor the by color, otherwise if the bg color is first, the bg image will override the bg color, only if the item with the bg color is inside the other item. Hope this makes sense.
I have created a sample site that has a background, with center-aligned content. you can download it as an attachment.
Feel free to use the site, but the background is not mine, found it on google.
I have also included a screenshot of the page.